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Chapter Objectives
Selecting Time Critical Message
Sets
Packet Format
Time Critical Message Set
In this chapter, you will read about the time-critical message set. This
chapter includes a description of each field of each protocol layer above
the HDLC data-link layer for time-critical messages.
Time-critical messages can read from and write to host devices at remote
stations. Also, time-critical messages can read immediate-access blocks
stored in interfaces of programmable controller stations. Any message
that is not in the time-critical message set is a supervisory message. If
two messages are waiting to be transmitted when a node gets the token on
the Data Highway II link, a time-critical message is given priority over
supervisory messages at that node. Therefore, the predictable response
time for time-critical messages is shorter than that for supervisory
messages; but you are limited to transferring 21 words maximum using a
read immediate-access block command and limited to transferring less
using other time-critical commands.
To enable the interface to send and receive time-critical messages initiated
by the host computer, you must set the communication-mode switches on
the host board to select the Native mode. (Refer to Chapter 4 for
switch-setting procedures).
Figure 9.1 shows a data frame containing a time-critical packet in three
layers:
The message fields at the application layer.
The message header fields common to all time-critical messages.
The fields common to all message transmissions in the Native mode.
Messages are initiated at the application layer and are placed into packets
in the network layer. For outgoing messages, the packet from the network
layer is passed down to the HDLC data-link layer and is sent out as the
data field of the data frame. For incoming messages, the data field from
the data frame is passed up from the HDLC data-link layer as a packet to
the network layer. In Chapter 6, we described the fields of the frame at
the HDLC data-link layer. Here, we describe the fields of the packet
passed up from and down to the HDLC data-link layer. At each layer
9
Chapter
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Troubleshooting
